The present invention relates to a transmission arrangement of the kind for converting the rotary movement of a motor into a linear curved movement of a driven body.
Transmission arrangements of that kind are used in particular as window lifting assemblies in motor vehicles, in which the doors, as viewed in the direction of travel, are curved convexly outwardly so that the windows must be moved along a curved path which follows the curvature of the door when they are opened or closed.
For that purpose it is known to arrange in the door a fixed electric motor driving a gear wheel or the like which engages into a cylindrical wire spiral or coil member and, upon being rotated, displaces it in the longitudinal direction. The wire spiral or coil member which has only a low degree of inherent stiffness transversely with respect to its longitudinal direction is so enclosed by a sheet metal tube which is slit in the longitudinal direction, that the wire spiral or coil member can only move back and forth in the tube. The one end of the wire spiral or coil member is rigidly connected to the window to be moved, while the element which connects those two parts together extends through the slit in the sheet metal tube. The part of the sheet metal tube in which the end of the wire spiral or coil member, which is connected to the window, moves back and forth, extends parallel to the path of movement of the window.
As only pulling or pushing forces can be applied by such an arrangement in the longitudinal direction of the sheet metal tube and the wire spiral or coil member which is guided therein, it is necessary to provide additional guide devices which carry frictional moments and lateral tilting moments of the member to be moved. That is difficult in particular when the window is to be guided only at one side, for design reasons.
So that the window stops in the respective position attained after the drive motor is switched off and cannot be pressed downwardly, generally the sheet metal tube which encloses the wire spiral or coil member is additionally wound to form at least one loop so that the cable friction as between the wire spiral or coil member and the sheet metal tube produces a self-locking effect which is independent of the motor or the engagement of the motor gear wheel into the wire spiral or coil member. However that cable friction has to be overcome in any deliberate lifting or lowering operation and therefore results in the consumption of an additional amount of power.
Another disadvantage of the known arrangement is that the sheet metal tube must be at least twice the length of the full lifting motion of the window so that the wire spiral or coil member continues to be guided in the sheet metal tube when the window is in the fully lowered position. That results in a comparatively large amount of space being required.